Composition of menstrual blood has been determined by various methods including examination of tissue fragments and cytology, measurement of hematological parameters, and determinations of components of coagulation and fibrinolysis in the menstrual blood. Tissue sampling has shown that lysosomes and hydrolytic enzymes are present in menstrual blood as methods of breaking down endometrial cells. Hematologically, menstrual blood differs from whole peripheral blood in that it has greater white cell counts, skewing the hemoglobin values; and the presence of vaginal secretions and transudation also affects the normality of hematological parameters. Many substances have been found or theorized to exist in menstrual blood which have specific roles in reproduction. Proteins that are specially synthesized for support of an ovum are an example (in rabbits, this protein is called uteroglobin, and is synthesized during the preimplantation phase of the cycle). Endometrial prostaglandins, as tissue stimulants for withdrawal bleeding, are another example. The endometrial content of enzymes is of utmost importance in the sequence of events leading to normal menstruation, and the content of enzymes in menstrual blood reflects the role of that organ in reproduction. Measurement of these enzymes from the endometrium may reveal the mechanism of tissue breakdown in menstruation. The historically considered menotoxin may be a function of bad sampling conditions, or may be a catch-all term for the presence of excessive enzymes involved in tissue brekdown or large amounts of prostaglandins synthesized for endometrial shedding. Menstrual blood loss (MBL) is affected by various contraceptive techniques. Combined pills reduce flow; progestin-only methods also diminish flow; IUDs increase MBL, according to clinical studies, probably because of increased uterine fibrinolysis.